Add text/event-stream for long running http connections (#7909)

When MinIO is behind a proxy, proxies end up killing
clients when no data is seen on the connection, adding
the right content-type ensures that proxies do not come
in the way.

cmd/bucket-notification-handlers.go

@@ -23,6 +23,7 @@ import (
 	"net/http"
 
 	"github.com/gorilla/mux"
+	xhttp "github.com/minio/minio/cmd/http"
 	"github.com/minio/minio/cmd/logger"
 	"github.com/minio/minio/pkg/event"
 	"github.com/minio/minio/pkg/event/target"
@@ -246,6 +247,8 @@ func (api objectAPIHandlers) ListenBucketNotificationHandler(w http.ResponseWrit
 		return
 	}
 
+	w.Header().Set(xhttp.ContentType, "text/event-stream")
+
 	target, err := target.NewHTTPClientTarget(*host, w)
 	if err != nil {
 		writeErrorResponse(ctx, w, toAPIError(ctx, err), r.URL, guessIsBrowserReq(r))